There have been broadly employed radiographic images such as X-ray images for diagnosis of the conditions of patients on the wards. Specifically, radiographic images using an intensifying-screen/film system have achieved enhancement of speed and image quality over its long history and are still used on the scene of medical treatment as an imaging system having high reliability and superior cost performance in combination. However, these image data are so-called analog image data, in which free image processing or instantaneous image transfer cannot be realized.
Recently, there appeared digital system radiographic image detection apparatuses, as typified by a computed radiography (also denoted simply as CR) and a flat panel detector (also denoted simply as FPD). In these apparatuses, digital radiographic images are obtained directly and can be displayed on an image display apparatus such as a cathode ray tube or liquid crystal panels, which renders it unnecessary to form images on photographic film. Accordingly, digital system radiographic image detection apparatuses have resulted in reduced necessities of image formation by a silver salt photographic system and leading to drastic improvement in convenience for diagnosis in hospitals or medical clinics.
The computed radiography (CR) as one of the digital technologies for radiographic imaging has been accepted mainly at medical sites. However, image sharpness is insufficient and spatial resolution is also insufficient, which have not yet reached the image quality level of the conventional screen/film system. Further, there appeared, as a digital X-ray imaging technology, an X-ray flat panel detector (FPD) using a thin film transistor (TFT), as described in, for example, non-patent documents 1 and 2.
To convert radiation to visible light is employed a scintillator panel made of an X-ray phosphor which is emissive for radiation. The use of a scintillator panel exhibiting enhanced emission efficiency is necessary for enhancement of the SN ratio in radiography at a relatively low dose. Generally, the emission efficiency of a scintillator panel depends of the scintillator layer (phosphor layer) thickness and X-ray absorbance of the phosphor. A thicker phosphor layer causes more scattering of emission within the phosphor layer, leading to deteriorated sharpness. Accordingly, necessary sharpness for desired image quality level necessarily determines the layer thickness.
Specifically, cesium iodide (CsI) exhibits a relatively high conversion rate of X-rays to visible light. Further, a columnar crystal structure of the phosphor can readily be formed through vapor deposition and its light guide effect inhibits scattering of emitted light within the crystal, enabling an increase of the phosphor layer thickness.
However, the use of CsI alone results in reduced emission efficiency. For example, there was disclosed a technique for use as an X-ray phosphor in which a mixture of CsI and sodium iodide (NaI) at any mixing ratio was deposited on a substrate to form sodium-activated cesium iodide (CsI:Na), which was further subjected to annealing as a post-treatment to achieve enhanced visible-conversion efficiency (as described in patent document 1).
There were also disclosed a technique in which a scintillator layer thickness of not less than 500 μm and a filling factor of columnar crystals in a scintillator layer of 70 to 80% achieved enhanced image resolution and high image quality without vitiating X-ray transmittance (as described in, for example, patent document 2); a technique in which a phosphor layer thickness of 300 to 700 μm and its relative density of 85 to 97% achieved enhanced sensitivity and sharpness (as described in, for example, patent document 3; and a technique in which reduction of layer thickness distribution or a coefficient of variation of layer thickness in a phosphor layer achieved reduced unevenness of sensitivity (as described in, for example, patent document 4).
Patent document 1: JP 54-35060B
Patent document 2: JP 2006-058099A
Patent document 3: JP 2002-214397A
Patent document 4: JP 2005-091140A
Non-patent document 1: the article, “Amorphous Semiconductor Usher in Digital X-ray Imaging” in Physics Today, November, 1997, page 24, and
Non-patent document 2: the article, “Development of a High Resolution, Active Matrix, Flat-Panel Imager with Enhanced Fill Factor” described in SPIE, vol. 32, page 2 (1997).